This invention relates generally to injection molding and more particularly to apparatus having a torpedo with a central shaft seated in the front end of a heated nozzle.
The use of torpedoes with conductive central shafts extending in the melt bore to enhance the thermodynamic cycle is well known. As seen in the applicants' U.S. Pat. No. 4,450,999 which issued May 29, 1984, it is also well known to make the torpedo shaft with an inner portion formed of a highly conductive metal such as copper covered by a protective casing formed of high speed steel. A torpedo having a similar shaft extending in alignment with a gate in a gate insert is shown in the applicants' U.S. Pat. No. 4,771,164 which issued Sep. 13, 1988. While these previous torpedoes have been successful for many applications, wearing of the shaft is a problem when the melt contains highly abrasive and corrosive materials such as ceramics, fiberglass, metals or minerals.